1. Field of the Invention
This invention relates to an SDMA (Space Division Multiple Acess)/TDMA(Time Division Multiple Access) satellite communication system.
2. Description of the Prior Art
Strictly speaking, the SDMA/TDMA satellite communication system is referred to as SDMA/SS-TDMA (Space Division Multiple Acess/Spacecraft Switched-Time Division Multiple Access). Such a system is viewed as a satellite communication system of large capacity based on the TDMA system.
With this system, a satellite contains a spot beam antenna which irradiates a relatively small zone unlike a conventional global beam antenna. Namely, the satellite contains several spot beam antennas and each spot beam corresponds to one zone without interference with the other beams. The same frequency is employed in common to the respective spot beams and a multiple access is achieved. The satellite contains a switching matrix having inputs and outputs corresponding to the respective spot beam zones. The transmission and reception of signals are achieved among the spot beam zones in accordance with a time sequence predetermined by clock pulses from a reference oscillator incorporated in the satellite. Further, in each spot beam zone, access is effected on the conventional TDMA system.
FIG. 1 shows a basic model of the SDMA/TDMA satellite communication system (hereinafter referred to as the SDMA/TDMA system, for the saake of brevity). In the case of FIG. 1, three spot beam zones No. 1, No. 2 and No. 3 exist and each spot beam zone covers three earth stations. Namely, earth stations No. 11, No. 12 and No. 13 belong to the spot beam No. 1; while earth stations No. 21, No. 22, No. 23 and No. 31, No. 32, No. 33 respectively belong to the spot beam zones No. 2 and No. 3.
The satellite contains a switching matrix SM, by which TDMA signals of the spot beam zones No. 1, No. 2 and No. 3 are suitably connected among them. FIGS. 2(a), 2(b) and 2(c) combine to illustrate an example of a basic time chart of one frame, including signal transmission and reception among the spot beam zones No. 1, No. 2 and No. 3. FIG. 2(a) shows a burst train which is applied to the switching matrix SM of the satellite from the spot beam zone No. 1. IN FIG. 2(a), reference character SB indicates a synchronization burst, derived fromthe spot beam zone No. 1 and returned thereto. Reference character DB designates data bursts, which show that signals from the earth stations No. 11, No. 12 and No. 13 are sequentially transmitted respectively to the spot beam zones No. 3, No. 1 and No. 2 in a time divisional manner. FIG. 2(b) shows the time sequence of switching (hereinafter referred to as the switching sequence) of the switching matrix SM of the satellite. In FIG. 2(b), reference character SW denotes a synchronization window, which is a time slot assigned for returning a synchronization burst of each spot beam zone to the spot beam zone from which it is transmitted. Reference character DW identifies data windows, which are time slots assigned for transmission and reception of data signals among the spot beam zones in a predetermined time sequence. FIG. 2(c) shows a burst train from the switching matrix SM of the satellite to the spot beam zone No. 1.
The feature of the SDMA/TDMA system resides in the frame synchronization established in synchronism with the switching sequence of the satellite. This synchronization is obtained by transmitting the synchronization burst SB from each of the earth stations of each spot beam zone to the synchronization window SW on the satellite and controlling the synchronization burst SB to bear a correct phase relation on the satellite. For the following reason, all of the earth stations of each spot beam zone are required to transmit the synchronization burst SB. Namely, in the SDMA/TDMA system, only the synchronization window SW provided on the satellite is guaranteed as the time slot for returning the synchronization burst of each spot beam zone but the data window DW which is the time slot other than the synchronization window SW is not always guaranteed as the time slot for the returning use. However, in FIG. 2, the time slot for the returning use is shown. Accordingly, unless the earth stations of each spot beam zone each transmit the synchronization burst SB towards the synchronization window SW which is the time slot for the returning use, frame synchronization cannot be obtained. Consequently, the synchronization bursts SB of all the earth stations of each spot beam zone access the synchronization window SW. The method therefore may be, for example, a frequency division or time division type.
FIG. 3 illustrates one example of the synchronization burst SB and the manner in which the synchronization is established in FIG. 2. It is called a PN-PSK(Pseudo Noise-Phase Shift Keying) synchronization signal. In FIG. 3, reference character PW indicates a preamble word for carrier reproducing and for bit timing reproducing; SIC designates a station identification cord; and MB identifies metric bits (Which imply bits to be measured). The decision of establishment of synchronization with the synchronization window SW, in the case of employing the synchronization burst (signal) SB, is achieved in the following manner. That is, the metric bits MB of the synchronization burst SB, which are composed of 2K bits, are overlapped on the synchronization window SW on the satellite in the vicinity of its fall. The difference between the number of bits from a first metric bit correctly received by the earth station to a first incorrectly received metric bit is relative to a measure of the phase error, and since only noises are received after the metric bits are cut off by the synchronization window SW, and the probability of erroneous reception increases. When the measured phase difference is zero, complete synchronization is established. Accordingly, FIG. 3 shows the state that complete synchronization is established. Where the measured phase difference is not zero, the synchronization burst SB has a phase difference relative to the synchronization window SW, so that it is necessary to control (correct) the transmit phase of the synchronization burst SB based on the measured value of phase error.
As described above, in the SDMA/TDMA system, for controlling the transmit phase of the synchronization burst SB of each spot beam zone, use is made of a time slot for returning use which is called the synchronization window SW of the switching sequence generated on the satellite. Namely, by correct synchronization of all the earth stations of each spot beam zone with the synchronization window SW on the satellite, normal communication among the spot beam zones is made possible.
The present invention concerns a system for controlling the transmit phase of the synchronization burst SB in such an SDMA/TDMA system as described above.
With a system for controlling the transmit phase of the synchronization burst SB of the conventional TDMA system, after the synchronization burst SB (of the station) phase-corrected at the preceding transmit phase control (correction) instant is received, a phase error measurement is achieved once and then phase correction is effected based on the measured value. Namely, in the TDMA system, frame synchronization of all the earth stations is accomplished by controlling the difference in receiving timing between the synchronization burst SB of a reference station of the earth stations and the synchronization burst SB of each station. However, in the SDMA/TDMA system the phase error measurement is achieved by utilizing the synchronization window SW of the switching sequence generated on the satellite, for example, by detecting the amount of the synchronization burst SB cut off the synchronization window SW, as described previously, so that frame synchronization among the spot beam zones is greatly affected by the rise/fall characteristic of the synchronization window SW and noises. Namely, in the example of FIG. 3, at a certain measuring instant, the respective metric bits are decided to be 1 or 0 for every bit. Even if only noise is received, the probability that the metric bit is decided to be 1 or 0 is 1/2. Accordingly, in the SDMA/TDMA system, even by effecting the phase error measurement after once receiving the synchronization burst SB of each station as in the conventional TDMA system, a decision error is inevitably introduced in the measurement and a correct phase error is not obtained. Therefore, it is impossible to achieve a correct transmit phase control of the synchronization burst.